1. Field of the Invention
The present invention relates to apparatus for non-invasively measuring one or more blood parameters. More specifically, the invention relates to apparatus for the transcutaneous measurement of vascular access blood flow. The invention can also be used for precise access location, as a xe2x80x9cflow finder,xe2x80x9d and also can be used to locate grafts and to localize veins in normal patients for more efficient canulatization.
2. Related Art
Routine determination of the rate of blood flow within the vascular access site during maintenance hemodialysis is currently considered an integral component of vascular access assessment. While the relative importance of vascular access flow rates and venous pressure measurements in detecting venous stenoses is still somewhat controversial, both the magnitude and the rate of decrease in vascular access flow rate have been previously shown to predict venous stenoses and access site failure. The traditional approach for determining the vascular access flow rate is by Doppler flow imaging; however, these procedures are expensive and cannot be performed during routine hemodialysis, and the results from this approach are dependent on the machine and operator.
Determination of the vascular access flow rate can also be accurately determined using indicator dilution methods. Early indicator dilution studies determined the vascular access flow rate by injecting cardiogreen or radiolabeled substances at a constant rate into the arterial end of the access site and calculated the vascular access flow rate from the steady state downstream concentration of the injected substance. These early attempts to use indicator dilution methods were limited to research applications since this approach could not be routinely performed during clinical hemodialysis. It has long been known that in order to determine the vascular access flow (ABF) rate during the hemodialysis procedure, the dialysis blood lines can be reversed (by switching the arterial and venous connections) to direct the blood flow within the hemodialysis circuit in order to facilitate the injection of an indicator in the arterial end of the access site and detect its concentration downstream (N. M. Krivitski, xe2x80x9cTheory and validation of access flow measurements by dilution technique during hemodialysis,xe2x80x9d Kidney Int 48: 244-250, 1995; N. M. Krivitski, xe2x80x9cNovel method to measure access flow during hemodialysis by ultrasound velocity dilution technique,xe2x80x9d ASAIO J 41: M741-M745, 1995; and T. A. Depner and N. M. Krivitski, xe2x80x9cClinical measurement of blood flow in hemodialysis access fistulae and grafts by ultrasound dilution,xe2x80x9d ASAIO J 41: M745-M749, 1995)). D. Yarar et al., Kidney Int., 65: 1129-1135 (1999), developed a similar method using change in hematocrit to determine ABF. Various modifications of this approach have been subsequently developed. While these latter indicator dilution methods permit determination of the vascular access flow rate during routine hemodialysis, reversal of the dialysis blood lines from their normal configuration is inconvenient and time-consuming since it requires that the dialyzer blood pump be stopped and the dialysis procedure is relatively inefficient during the evaluation of the flow rate which can take up to twenty minutes. Furthermore, some of these indicator dilution methods also require accurate determination of the blood flow rate.
Clinical usefulness and ease of use are major developmental criteria. From a routine clinical point of view the need to design a simple sensor, easily attached to the patient, requiring no line reversals, no knowledge of the dialysis blood flow rate, Qb, and transcutaneously applied to skin, thereby accomplishing the measurement within a total of 1-2 minutes, is crucial to have repeated, routine meaningful ABF trend information, whereby access health is easily tracked.
It is therefore an object of the present invention to provide apparatus for non-invasively measuring one or more blood parameters.
It is another object of the present invention to provide an optical hematocrit sensor that can detect changes in hematocrit transcutaneously.
It is still another object of the invention to provide an optical hematocrit sensor that can be used to determine the vascular access flow rate within 2 minutes and without reversal of the dialysis blood lines or knowledge of Qb, all transcutaneously.
These and other objects of the invention are achieved by the provision of an optical sensor including complementary emitter and detector elements at multiple spacings (d1, d2) for the purpose of measuring the bulk absorptivity (xcex1) of the volume immediately surrounding and including the access site, and the absorptivity (xcex1o) of the tissue itself.
In one aspect of the invention, the optical sensor system comprises an LED of specific wavelength and a complementary photodetector. A wavelength of 805 nm-880 nm, which is near the known isobestic wavelength for hemoglobin, is used.
When the sensor is placed on the surface of the skin, the LED illuminates a volume of tissue, and a small fraction of the light absorbed and back-scattered by the media is detected by the photodetector. The illuminated volume as seen by the photodetector can be visualized as an isointensity ellipsoid, as individual photons of light are continuously scattered and absorbed by the media. Because a wavelength of 805 nm-880 nm is used, hemoglobin of the blood within the tissue volume is the principal absorbing substance. The scattering and absorbing characteristics are mathematically expressed in terms of a bulk attenuation coefficient (xcex1) that is specific to the illuminated media. The amount of light detected by the photodetector is proportional via a modified Beer""s law formula to the instantaneous net xcex1 value of the media.
When the volume of tissue illuminated includes all or even part of the access, the resultant xcex1 value includes information about both the surrounding tissue and the access itself. In order to resolve the signal due to blood flowing within the access from that due to the surrounding tissues, the sensor system illuminates adjacent tissue regions on either side of the access. Values of xcex1o for tissue regions not containing the access are then used to normalize the signal, thus providing a baseline from which relative changes in access hematocrit can be assessed.
Other objects, features and advantages of the present invention will be apparent to those skilled in the art upon a reading of this specification including the accompanying drawings.